The present invention relates to the art of well perforation and, more particularly, concerns a novel method and apparatus for drilling new and/or extending existing perforation holes within existing or new oil and gas wells or similar excavations.
It is conventional practice after a well has been drilled and cased to its desired depth, to perforate a bore hole with one or more 3/8" to 1" diameter holes at the depth of the lowest most promising formation. For each perforation to be made, a projectile is discharged at a velocity sufficient to cause it to penetrate, or burn a hole through, the well casing and cement and out to approximately 18 inches into the formation.
Promising oil bearing formations vary vertically from five feet to as much as several hundred feet. Normal practice is to perforate one to four holes (through the steel casing) for every vertical foot of promising formation. The depth to which the steel casing is normally set extends approximately fifty feet below the level of the bottommost perforation to what is normally termed the bore hole bottom. The production string of casing is then cemeted using a float collar and a guide shoe to support the cement column behind (outside) the casing to a point above the highest point to be tested for production. A packer is next run on tubing and set in the casing approximately ten feet above the top perforation. This packer is designed mechanically to fit pressure tight against the inner sidewalls of the casing and is normally not moved, once set. It has a circular center opening and it is through this opening that the geled water and sand mixture or acid, known as the fracing or treating solution, is passed on its way into the perforated area and the oil bearing formation through the perforation holes made by the bullet or jet shots.
The pump pressure behind this fracing or treating solution varies from a few hundred p.s.i. to several thousand p.s.i. depending upon how much pressure is required to open up the formation so it will accept the fracing or treating solution.
Once the fracing solution has, under high pressure, transported the sand into the openings (fractures) in the formation and lodged the sand there, the high fracing pressure is released and the geled solution and the hard sand separate. After this release of the high fracing pressure, the separated gelled solution either flows back into the well bore hole or is carried there by the first flow of oil as the latter comes out of the formation and flows into the well casing on its way to the surface.
Because the horizontal orientation of the perforation producing projectile cannot accurately be predetermined or controlled from the surface, the resulting pattern of perforations created at the formation level has heretofore been essentially unpredictable. This, of course, is disadvantageous because the formation may not be perforated in the proper direction to maximize recovery flow. Similarly, as previously indicated, present methods permit only limited penetration by the perforation bullet into the formation (nominally up to 18") thereby often limiting the area of the pay zone from which recovery flow can be achieved.
Utilization of the laser energy has been heretofore applied in earth boring applications. For example, applications involving use of a laser power capability in the region of 10,000 kilowatts are disclosed in Salisbury and Stiles U.S. Pat. Nos. 3,998,281 and 4,066,138, the disclosures of which are incorporated herein by this reference. Laser energy generators of such high continuous power capability are, at the present state-of-the-art, physically large, comparatively heavy and bulky and not subject to use in confined quarters such as a conventional well bore hole. This is true also of smaller single unit laser energy generators of low to medium power such as those currently avaiable for unclassified use, in the 1 kilowatt to 200 kilowatt continuous power range.
Utilization of laser energy from surface mounted laser energy generators for the purpose of drilling perforation holes and/or the lengthening of existing perforation holes deep within a well's bore hole is further complicated by the fact that existing oil wells are not normally drilled in optically straight lines due to the fact that the bedding plane of the rock layers is not flat, causing the bit to drift with the dip of the rock formation. Laser energy, which does travel in an optically straight line, will not follow such bore hole drifting without some form of high efficiency laser energy channeling (laser transmission line).